5. Republicans need to pick up 4 seats to regain control of the U.S. Senate. They can also recapture control by winning the presidency in 2012 and picking up 3 Senate seats (the newly elected Vice President would give them their decisive vote).

Cook notes that it's far too early to make any kind of precise projection as to the partisan composition of the next U.S. Senate. But he anticipates a wave of incumbent retirements. Three sitting Senators -- Conrad (D-ND), Hutchison (R-TX) and Lieberman (I-CT) -- have already announced they will not seek re-election. There will likely be three or four more retirement announcements this year.